The present invention relates to a demodulation system for frequency- or phase-modulation signals, and more specifically relates to an automatic frequency control system for use in digital demodulation of the frequency- or phase-modulation signal by using quadrature-phase.
Generally, a frequency modulation (FM) receiver employs primarily a superheterodyne system, in which a double frequency conversion is executed that is different from a conventional superheterodyne system, as shown in FIG. 1. Referring to FIG. 1, a conventional automatic frequency control system will be explained with reference to the superheterodyne system of a double frequency conversion which includes first and second local oscillators (3,6), first and second mixers (4,7), and the first and second IF (Intermediate Frequency) filters (5,8). When a radio frequency (hereinafter referred to as RF) signal modulated in frequency or phase is received through an antenna 1, it is applied to the first mixer 4 through a low-noise amplifier 2. At this moment, the first mixer 4 produces a first IF signal by mixing the received RF signal and a first local oscillation signal (LS1) generated from the first local oscillator 3, and the first IF filter 5 eliminates undesirable components such as image-frequency and higher-harmonics thereof resulting from the mixing in the first mixer and thereby detects only the IF signal (IF1). The second mixer 7 produces a second IF signal by mixing said first IF signal (IF1) and a second local oscillation signal (LS2) generated from the second local oscillator 6, and the second IF filter 8 outputs the second IF signal (IF2) and thereby eliminates any adjacent frequencies in order to take extremely sharp discrimination against adjacent channels. Here, ordinarily the first IF signal is a high frequency of several tens of MHz band and the second IF signal is a relatively low frequency band. A FM demodulator 9 reproduces the original signal by demodulating the second IF signal (IF2). When a frequency deviation is present in the first local oscillator 3 and second local oscillator 6, the output of the FM demodulator 9 produces a direct-current (DC) output proportional to said frequency deviation. In order to compensate this, the output of the demodulator 9 is applied to a low-pass filter (LPF) 10 whose cut-off frequency is considerably low, and the LPF 10 eliminates the alternating-current (AC) components from the demodulation signals, thereafter filtering only the direct-current signals to a voltage controlled oscillator 11.
Therefore, since the first local oscillator 3 is synchronized with the output of the voltage controlled oscillator 11, if the voltage controlled oscillator 11 is controlled toward the direction of offsetting the direct-current component according to the deviation of frequency, the first local oscillation signal (LS1) tracks the RF signal received. However, since the direct-current signal passed through the demodulator 9 is proportional to a sum of deviations of the local oscillation frequencies LS1 and LS2 of the first and second local oscillators 3 and 6, it operates mostly with an error against the frequency deviation of the second local oscillator 6. Further, since the LPF is an analog circuit having a low cut-off frequency, it is impossible to ideally eliminate the alternating-current component of the demodulation signal. Also, when the signal of such an alternating-current component is fed back to the first local oscillator 3, the first local oscillation frequency becomes unstable and is apt to include noise and the like. Moreover, there has been a problem that a part of such signals is modulated into the output in case of a full-duplex system.